Electrochemical cell with circumferential cathode current collector

ABSTRACT

An electrochemical cell has a silver vanadium oxide cathode material formed into a pellet shape which expands as the cell is discharged. A cathode current collector circumferentially surrounds the cathode pellet and is in contact with the peripheral edge of the cathode pellet to prevent peripheral cathode expansion. The peripheral cathode current collector maintains a stable cell impedance during cell discharge. The cell has a D-shaped housing in which the cathode is disposed adjacent to a first interior surface, and a lithium anode is disposed adjacent to a second interior surface.

This application is a continuation of U.S. patent application Ser. No.08/882,505 filed Jun. 25, 1997 now abandoned, entitled "ElectrochemicalCell" to Sunderland et al., which is a division of U.S. patentapplication Ser. No. 08/638,624 filed Apr. 26, 1996, now U.S. Pat. No.5,716,729.

BACKGROUND OF THE INVENTION

This invention relates to an electrochemical cell, a current collectorfor the cell and a method for cell construction. In particular, theinvention relates to a hermetically sealed cell having a reactivecathode and a lithium anode. The electrochemical cell may be used topower body implantable medical devices such as heart pacemakers.

In constructing an electrochemical cell for use in implantable medicaldevices, a known method of making a cathode pellet is to compress amixture of powdered metal oxide, a conductive matrix such as graphite orcarbon black and a binding material such as polytetrafluroethylene(PTFE). In such a cell, it is essential for uniform discharge of thecell to maintain good contact between the cathode current collector andthe cathode material. One known way of providing such contact is toimbed the current collector inside the cathode powder mixture and thencompress the mixture into a pellet.

A drawback of imbedding the current collector in the cathode material isthat the volume of the cathode typically expands as the cathode isdischarged. This expansion of the cathode can cause degradation of thecontact between the cathode and the current collector, causing changesin overall cell impedance as the cell is discharged. During latterstages of discharge, contact degradation may cause such an increase inimpedance that it causes a significant decrease in cell capacity. Sincehigh reliability in a surgically implanted medical device is essential,an unexpected reduction in cell capacity means that the device must besurgically removed and replaced much earlier than usual.

It should be understood that not all cells with imbedded currentcollectors will undergo a dramatic impedance increase that requiresreplacement of the cell. However, even without the problem of early cellreplacement, the variability of cell impedance complicates the use ofthe cell. Powering critical implantable medical devices such aspacemakers, neurostimulators and drug infusing pumps, requirescompactness and efficiency of circuit design. If the cells used are toovariable in their output, the device would need to be larger and lessefficient since the circuits must either increase in complexity tocompensate for the output variability or must include capacitors whichcan provide additional energy storage.

Of course, it is known to provide cathodes without imbedded currentcollectors. For example, U.S. Pat. No. 3,440,110 issued to Arbterdiscloses a cathode assembly which includes a support ring into which acathode material is pressed. This cathode assembly is then pressed intointimate contact with the bottom of the cell housing so that the housingitself can contact the cathode material and act as a current collector.However, in a high reliability electrochemical cell for use in criticalmedical device applications, reliance on this contact between thecathode and the case can still raise a concern about undesirableimpedance variations.

It is, therefore, an object of the present invention to provide anelectrochemical cell in which a cathode material which is subject toswelling during discharge has a current collector which will obviatecell impedance variability.

SUMMARY OF THE INVENTION

We have discovered an electrochemical cell having a metal housing and ananode and cathode within the metal housing in which the cathode assemblyincludes a ring-shaped current collector into which the cathode materialis pressed. The cathode is formed into a pellet shape with a flat topsurface, a flat bottom surface and a peripheral edge extending betweenthe top and bottom surfaces. The cathode current collectorcircumferentially surrounds the cathode pellet and is in contact withthe peripheral edge of the cathode pellet. Since the cathode iscomprised of a material which expands as the cell is discharged, theexpansion of the cathode material against the confining ring-shapedcurrent collector will serve as a stable connection between the currentcollector and the cathode material. The current collector is thenelectrically connected to the metal housing to allow current flowbetween the current collector and housing. The housing may therefore beused as one terminal of the cell.

In one aspect of the invention, the cathode current collector can be aring having an open top portion exposing the top surface of the cathodepellet and an open bottom portion exposing the bottom surface of thecathode pellet. Since neither the top nor bottom portions of the cathodepellet are confined by the current collector, the expansion of thecathode material during discharge may be distributed more evenly on bothsides of the ring. Also, another advantage for the open ring shape isthat it allows the entire thickness of the cathode assembly to be filledwith reactive cathode material to maximize cathode capacity in the cell.

In yet another aspect of the invention, the cathode current collectorcan have a non-circular shape such as a D-shape in which the ring isreinforced in order to maintain its shape during cathode discharge. Thereinforcement can be, for example, a flange extending around the ring orplaced selectively at portions of the ring which are susceptible todeformation. Typically, the is flange would be provided at the topportion or the bottom portion of the current collector ring. Preferably,the flange extends inwardly around the ring to assist in the retentionof the cathode pellet but still exposing the center portion of thecathode pellet.

In yet another aspect of the invention, the metal housing of the cell isprovided with an insulator material which electrically separates thecathode material from the metal housing. The ring-shaped currentcollector then provides the connection to the housing. A connector tabcan extend outwardly from the cathode current collector to a locationremote from the cathode material where it can be attached to the metalhousing by welding or some other means for providing electrical contact.

In yet another aspect of the invention, the ring-shaped currentcollector allows for construction of the electrochemical cell by aconvenient method. The current collector is formed and placed into acircumferentially fitting die where the cathode material can be pressedinto a self-supporting pellet which is retained within the cathodecurrent collector. The combined cathode current collector and cathodematerial can then be placed as a unit into the metal housing of the celland electrically connected to a portion of the metal housing. Also, aportion of the flange of the current collector ring can be provided withan outward bend to provide a tab to secure the current collector to themetal-housing at a portion of the housing that is not insulated. Thiswould also have the effect of making a stable electrical connectionbetween the current collector and the housing. This method isparticularly useful when assembling a cell with a pocket-like, D-shapedhousing since the current collector assembly can be readily insertedinto the pocket of the housing and secured to the housing by welding atthe pocket opening.

In yet another aspect of the invention, the cathode current collectorallows for the use of an alternative connection to a feedthrough pinwhich would allow the metal housing of the cell to be used as a negativeterminal for the cell (i.e. connecting the anode to the metal housingand the cathode to a feedthrough pin) or for the housing to be neutralwith respect to the terminals of the cell (i.e. the anode and cathodeeach connected to a feedthrough pin).

BRIEF DESCRIPTION OF DRAWINGS

The invention will be further described with reference to theaccompanying drawings, in which:

FIG. 1 shows a cross-sectional, elevational view of the cell.

FIG. 2 shows a cross-sectional, side view of the cell along line 2--2 ofFIG. 1.

FIG. 3 shows an elevational view of the non-circular ring currentcollector of FIG. 1.

FIG. 4 shows a cross-sectional view, along line 4--4, of the currentcollector of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a cell 1 according to the invention is shown,including a metal housing in two parts, a D-shaped cell housing body 10and a housing cover 15, both comprised of a metal such as stainlesssteel or titanium. The housing cover 15 is placed in the open end ofhousing body 10 and is hermetically sealed to the housing body 10 with aloser weld around the entire edge of the housing cover 15. The housingcover 15 has an opening 20 to allow a conducting pin 25 to beexternalized through a feedthrough with a glass seal 28 which insulatesthe pin 25 from the housing cover 15 and hermetically seals the opening20. The housing cover 15 also has a fill port 30 which allows filling ofthe cell 1 with an electrolyte after the housing cover 15 is welded tothe housing body 10. After the cell 1 is charged with a liquidelectrolyte, a disc 35 is welded into the fill port 30 to provide ahermetic seal. The liquid electrolyte charged into the cell 1 caninclude an organic solvent in combination with an ionizing solute. Theorganic solvent can be, for example, diethyl carbonate,dimethytcarbonate, butylene carbonate, 3-methyl-2-oxazolidone,sulfolane, tetrahydrofuran, methyl-substituted tetrahydrofuran,1,3-dioxolane, propylene carbonate (PC), ethylene carbonate, gamma-butyrolactone, ethylene glycol sulfite, dimethylsulfite, dimethylsulfoxide or mixtures thereof and also, for example, low viscositycosolvents such as tetrahydrofuran (THF), methyl-substitutedtetrahydrofuran(Met-THF), dioxolane (DIOX), dimethoxyethane (DME),dimethyl isoxazole (DMI), diethyl carbonate(DEC), ethylene glycolsulfite (EGS), dioxane, dimethyl sulfite (DMS) or the like. The ionizingsolute can be a simple or double salt or mixtures thereof, for example,LiBF₄, LiAsF₆, LiPF₆, LiClO₄, LiCF₃ SO₃, Li(SO₃)(CF₃)₃, LiN(SOCl₂)₃, orLiC(SO₂ CF₃)₂, which will produce an lonically conductive solution whendissolved in one or more solvents.

FIG. 2 shows various elements contained within the housing body 10. Ananode layer 40 of an active metal is pressed onto a thin anode currentcollector 45 comprised of a conducting metal such as stainless steel,nickel or titanium. One end of the pin 25 is attached by welding to theanode current collector 45. Active metal anode materials can include,for example, aluminum, the alkali metals, alkaline earth metals andalloys of alkali metals or alkaline earth metals with each other andother metals. The preferred anode materials are lithium, sodium,potassium, calcium and alloys thereof.

A cathode assembly, including a cathode pellet 55 and cathode currentcollector 60 is spaced apart from the anode layer 40 by separator 50which is comprised of a porous or a microporous material, preferablypolypropylene or polyethylene. The separator 50 completely surrounds andseals the anode 40 and anode current collector 45. The reactive cathodematerial is a material which will swell upon discharge. Manganesedioxide is one such material. Other suitable cathode materials could beused instead of manganese dioxide, including vanadium oxide (V₂ O₅),silver vanadium oxide (Ag₂ V₄ O₁₁) carbon monoflouride, CoO₂, NiO₂, andTiS₂. The cathode pellet 55 may include binders and conductivityenhancers in addition to the reactive cathode material. Binders whichmay be typically employed in the cathode of the present invention arepolytetrafluoroethylene, ethylene/propylene copolymers and the like.Representative of the conductive materials which may be employed as aconductivity enhancer are graphite, carbon and the like. In a cathodepellet 55 having manganese dioxide as the reactive component, bindersmay comprise between about 1 and about 10 weight percent, preferablybetween about 1 and about 5 weight percent, of the cathode mix used tomake the cathode pellet 55 while the conductive material may comprisebetween about 1 and about 12 weight percent, preferably between about 3and about 10 weight percent, of the cathode mix. The solid cathodematerials used to make the cathode pellet 55 are in finely go dividedform so they can be intimately mixed. The cathode mixture may then bepressed into the cathode current collector 60 such that a selfsupporting cathode pellet 55 is formed within the current collector withperipheral edges of the cathode pellet 55 pressed into intimate contactwith the current collector 60. The intimate contact of the currentcollector 60 around the cathode pellet 55 has the effect of confiningthe cathode pellet 55 from expanding in diameter as the cathode pellet55 expands in volume during cell discharge.

A lining material 65, such as polyethylene, electrically insulates theanode 40 and cathode pellet 55 from the interior of the housing body 10.The lining material 65 can be provided in a thin, molded pocket-shapeditem or a porous or microporous material which fits closely within thehousing body 10. The cathode current collector 60 is electricallyconnected to the housing body 10 at connector tab 75 which is welded tothe housing cover 15.

Referring now to FIG. 3, the cathode current collector 60 has agenerally D-shaped outline that is partly semicircular and partlypolygonal. The cathode current collector 60 is comprised of a wall 80with a rim portion 70 and an flange portion 65. The purpose of theflange 65 is to provide a reinforcing means which will retain the inshape of the cathode current collector 60 as the cathode pellet expands.Accordingly, the reinforcing means may also include internal or externalflanges or a thickened portion of the cathode current collector 60 orother known means for retaining a shape. As in FIG. 3, the currentcollector 60 preferably has a uniform width, as measured from the rimportion 70 to the inward flange 65.

The cathode current collector 60, comprising an electrically conductivematerial such as stainless steel or titanium, can-be formed by stampinga metal sheet into a cup, cutting excess steel or titanium sheet aroundthe cup but leaving a rectangular tab on a straight edge of the cup. Anaperture can then be punched out at the bottom of the cup forming theinward flange 65 portion of the curent collector 60. The rectangular tabcan then be bent perpendicularly to create the electrical connector tab75 of the current collector 60.

After the cathode current collector 60 is formed, a collector-cathodeassembly is made. The cathode current collector 60 is placed in aclosely fitting die fixture such that the die maintains the shape of thecathode current collector 60 as the collector-cathode assembly is made.A measured amount of cathode mixture comprising powdered manganesedioxide, an inert binding material such as PTFE and conductivityenhancer such as graphite or carbon black is placed into the die insidethe current collector. The cathode mixture is compressed in a presswithin the cathode current collector 60 to form a self-supportingcathode pellet 55 having opposite, flat surfaces exposed. It will beappreciated by those skilled in the art that other active cathodematerials which cause the cathode pellet 55 to expand as the cell isdischarged could be used in in place of the manganese dioxide. Forexample, MnO₂, CF_(x), V₂ O₅, and a silver vanadium oxide (e.g. Ag₂ V₄O₁₁) could be used alone or in combination as active cathode materialsin the present invention.

FIG. 4 provides a cross-sectional view of the cathode current collector60, including the wall 80 with rim portion 70 and inward flange portion65. The connector tab 75 has a flat surface 85 which is electricallyconnected to the housing cover 15.

As an alternative embodiment of the invention, the connector tab 75 onthe cathode current collector 60 can be electrically connected to thehousing 10 or the cover 15 via a separate, intervening conductor. Oneend of the conductor may be electrically connected to the housing cover15. The other end of the conductor is electrically connected to theconnector tab 75 of the cathode current collector. The electricalconnections can be made by welding.

In yet another embodiment of the invention, a case negative version ofthe cell (i.e. with the case at the anode potential) can be easily madeby connecting the anode current collector 45 to the metal housing cover15 by a tab similar to the connector tab 75 on the cathode currentcollector 60 and the anode curent collector to the conducting pin 25.Also, a case neutral version of the cell (i.e. with the case at neitherthe anode or cathode potential) can be made by adding a secondconductive pin to the feedthrough or adding a second feedthrough so thateach of the current collectors 45, 60 can be connected to a separateconductor pins passing through the metal housing.

It will be appreciated by those skilled in the art that while theinvention has been described above in connection with particularembodiments and examples, the invention is not necessarily so limitedand that numerous other embodiments, examples, uses, modifications anddepartures from the embodiments, examples and uses may be made withoutdeparting from the inventive concepts.

We claim:
 1. A case-positive electrochemical cell, comprising:(a) aD-shaped titanium housing having first and second substantially parallelopposing interior surfaces spaced apart from one another; (b) a lithiumanode disposed between the first and second surfaces and adjacent to thefirst interior surface, (c) a cathode in operative relation to theanode, the cathode comprising silver vanadium oxide cathode material,the cathode being formed into a non-circular pellet shape having anouter periphery and disposed between the first and second surfaces andadjacent to the second interior surface; (d) a cathode current collectorcircumferentially surrounding the cathode pellet, the cathode currentcollector confining the outer periphery of the cathode as the cathodeexpands upon electrical discharge of the cell; (e) means forelectrically connecting the cathode current collector to the titaniumhousing; and (f) an insulator member insulating the titanium housingfrom contact with the cathode pellet.
 2. An electrochemical cellaccording to claim 1 wherein the cathode current collector is anon-circular ring having an open top portion exposing the top surface ofthe cathode pellet and an open bottom portion exposing the bottomsurface of the cathode pellet, and reinforcement means for retaining thenon-circular shape of the current collector.
 3. An electrochemical cellaccording to claim 2 wherein said reinforcement means comprises a flangeat the top portion or bottom portion of the current collector.
 4. Anelectrochemical cell according to claim 3 wherein the flange is inwardlyextending and forming an exposure of a portion of the cathode pellet. 5.An electrochemical cell according to claim 1 wherein said means forelectrically connecting includes a connector tab extending outward fromcontact with the cathode current collector.
 6. An electrochemical cellaccording to claim 5 wherein the means for electrically connectingincludes a weld extending between the connector tab and the metalhousing.
 7. An electrochemical cell according to claim 1, wherein thecell further comprises an electrolyte having an aprotic organic solventand a solute selected from the group consisting of LiCF₃ SO₃, Li(SO₃)(CF₃)₃, LiBF₄, LiAsF₆, LiPF₆, LiClO₄, imide and methide.